As shown in FIG. 1, commercial aircraft 1 typically carry cargo on pallets 2 and in large containers called unit load devices (ULDs) 4. Standardized ULDs 4 and pallets 2 are available in several different configurations having different lengths and widths. All ULDs 4 and pallets 2 must be securely retained during flight to prevent damage to the aircraft 1 and the cargo and to maintain aircraft stability and balance during flight. Accordingly, various types of restraint devices are used to laterally and vertically restrain ULDs 4 and pallets 2 in their stowed positions within an aircraft's cargo compartments. Such restraining devices are typically mounted to an aircraft's cargo deck structure 7 at fixed points that correspond to the lower edges of stowed ULDs 4 and pallets 2. Based upon the particular types and sizes of ULDs 4 and pallets 2 to be stowed onboard an aircraft, the positions of the restraint devices on the aircraft's cargo deck 7 can vary.
In some cases, multiple restraint devices are installed at points that will accommodate various combinations of ULD and pallet types and sizes. For example, centerline restraints can be positioned along the longitudinal centerline 6 of a cargo deck 7, and side restraints can be positioned in rows that are parallel to and at various distances from the centerline 6, including along the sidewall 8 of a cargo compartment. For a ULD 4 that is about half as wide as an aircraft's cargo hold, a ULD 4 can be loaded and stowed such that an inboard lower edge of the ULD 4 is vertically and laterally restrained by one or more centerline restraints positioned along the aircraft's centerline 6, and an opposed exterior lower edge of the ULD 4 is vertically and laterally restrained by one or more side restraints positioned adjacent to the aircraft's sidewall 8. For a wide ULD 4 or pallet 2 that is stowed along an aircraft's centerline 6, opposed lower outboard edges of the ULD 4 or pallet 2 can be vertically and laterally restrained by opposed sets of side restraints positioned adjacent to the aircraft's opposed sidewalls 8, for example.
Various types of restraint devices have been developed to accommodate various cargo configurations and to facilitate cargo loading and unloading. For example, some restraint devices are designed to be manually retracted to a stowed position when not used for a particular load configuration such that the devices will not interfere with the loading and unloading of ULDs. Some restraint devices also are overrideable such that they automatically retract to a stowed position when contacted and overridden by a ULD as the ULD is being moved along a cargo deck. Vertical cargo restraint devices typically include restraint heads that extend over a protruding lip or edge along the base of a ULD, thus blocking upward displacement of the ULD. Because restraint devices are typically positioned along the edges of cargo lanes that define rows of stowed ULDs, such restraints can also function to guide ULDs along these lanes as the ULDs are moved along a cargo deck during loading and unloading. These restraints can also block lateral movement of stowed ULDs.
In some aircraft, flexing of the aircraft's wings during flight can causes temporary changes to the aircraft's cargo deck structure, particularly in the area of the aircraft's wing box. For example, as shown in FIG. 2A, downward flexure of an aircraft's wings can cause upward bowing of the aircraft's cargo deck structure. In an extreme case, this upward bowing can cause outboard side vertical restraints attached to the cargo deck to temporarily pull away and possibly disengage from their associated ULDs.
Conversely, as shown in FIG. 2B, upward flexure of an aircraft's wings can cause downward bowing of an aircraft's cargo deck structure. Such downward bowing of the cargo deck can cause stowed ULDs to be squeezed between their restraints, and can result in substantial lateral loads on the restraint devices. In an extreme case, such lateral loads on the restraints may overload the aircraft's supporting floor structure.
Typically, cargo restraints are either fixed in their operable upright positions (i.e. not overrideable), or are designed to resiliently retract from their upright positions when overridden by ULDs. A restraint that is fixed in its upright position can be susceptible to damage from contact with a ULD if an operator neglects to retract the restraint when the restraint is not used for a particular cargo load configuration. Overrideable restraints can prevent such damage, but can also be prone to unwanted retraction upon initial contact with a compatible ULD. In such a case, the spring or springs used to upwardly bias the head of an overrideable restraint may be incapable of adequately resisting unwanted retraction of the restraint upon contact with a compatible ULD. Such unwanted retraction of an overrideable restraint can prevent the restraint from guiding the compatible ULD as the ULD traverses an associated cargo lane, and can preclude restraining engagement between the restraint and the compatible ULD. When an active restraint is inadvertently pulled down in this manner due to contact with a compatible ULD, the restraint and/or the ULD can be damaged.
Accordingly, there is a need for a vertical cargo restraint that can accommodate being squeezed against a ULD without transmitting damaging loads to an aircraft's deck structure. In addition, there is a need for a vertical restraint device that will remain engaged with a ULD even as flexure of an associated deck structure acts to pull the restraint away from the ULD. Furthermore, there is a need for an overrideable vertical cargo guide and restraint that will automatically lock in an erect position when the guide and restraint is engaged by a compatibly-sized ULD, and will also automatically retract when overridden by an incompatible ULD.